Wing Spar Sag and when to use Cradle?

[CharlieWaffles]

So I built my wing cradles today and was looking through the wing plans and see that in section 13 (Main Spar) it says the spar will sag under it's own weight. But then in section 16 (top wing skin), it first mentioned the craddle/straddling of the spar. But it doesn't look like the spar has been reinforced before then. Does all the early drilling/prep work (sections 13-15) need to occur on a work surface that is fully supporting the spar or can all that work be done in the craddle with 2x4s on both end (no carpet roll yet)?

 

[flion]

Or on saw horses. Yes, the only critical step is drilling the J-stringers (and I had some thoughts about that which will go in another thread). The ribs (section 14) obviously don't care if there is a slight bow in the spar. Likewise, the rear spar is flexible enough to allow you to attach it to the ribs and even begins stiffening up the structure slightly. Once you've clecoed the top skins on, any remaining sag will be gone and you can match drill safely. I just realized the other day that I could put the spar box on the saw horses, bottom up, and put the spar on top of that and all sag will be eliminated until I am ready to put the wings in the cradle. We're not talking a lot of sag anyway; with the spars supported at the very ends, I got maybe 1/8" (that's with no other structure). Moving them so that on is about 1/8 span from the root end and the other about 1/4 span from the tip eliminates virtually all the sag (again, without other structure yet). But now that I've thought of it, I'll build over the spar box until the top skins are on - just to be anal about it.

 

[rleffler]

To be honest, I don't know. I had two standard EAA tables that I used to build the wings. Obviously no sag with using a couple benches.

bob

 

[paul330]

I lay my spar on 3 saw horses and shimmed it to level both ways using a digital level. Got it to within 0.1 degree.....

 

[9GT]

I had no sag and just built the wings on the table per the plans. The pre-drilled holes in the ribs, spar and skins pull everything into alignment. I guess a jig is needed on the other models but not for the -10.